1. Field of the Invention
The present invention relates in general to a hydraulically operated braking system for a motor vehicle, and more particularly to improvements in the operating reliability of such a vehicle braking system including a pressure control valve device adapted to regulate a fluid pressure to be applied from a fluid pressure source such as a master cylinder to a wheel brake cylinder for braking a wheel of the vehicle.
2. Discussion of the Related Art
There is known a hydraulically operated braking system for a motor vehicle, as disclosed in JP-A-139279, for example, which system includes 1) a wheel brake cylinder for braking a wheel of the vehicle, 2) a hydraulic pressure source for pressurizing a working fluid according to an operation of a brake operating member, so that the pressurized fluid is supplied to the wheel brake cylinder, and 3) a pressure control valve device which is adapted to regulate the pressure of the fluid to be applied to the wheel brake cylinder, and which has at least a pressure-increasing state for allowing a flow of the pressurized fluid from the hydraulic pressure source to the wheel brake cylinder, and a pressure-reducing state for allowing a flow of the pressurized fluid from the wheel brake cylinder.
The hydraulic pressure source may include a constant pressure source automatically driven to generate a pressurized fluid having a substantially constant pressure, and/or a variable pressure source adapted to pressurize a fluid in a master cylinder such that the pressure of the pressurized fluid changes with an operating force or amount of a manually operated brake operating member. The pressure control valve device is preferably constructed to have a pressure holding state in addition to the pressure increasing state and the pressure reducing state. In the pressure holding state, the flows of the fluid to and from the wheel brake cylinder are both inhibited. The braking system is usually provided with a reservoir for storing the fluid discharged from the wheel brake cylinder.
In the hydraulic braking system of the type described above, the pressure control valve device is capable of regulating the fluid pressure in the wheel brake cylinder, such that the level of the fluid pressure in the wheel brake cylinder does not directly depend upon the operating state (operating force or amount) of the brake operating member. In a motor vehicle equipped with an electric motor as a drive power source, for instance, the hydraulic braking system may be operated in cooperation with the electric motor placed in a regenerative braking mode. Described in detail, the electric motor placed in the regenerative braking mode is operated as an electric generator or dynamo so as to provide a regenerative braking system, and the electric energy generated by the electric generator is stored in a battery. However, the braking force generated by the regenerative braking system is often insufficient to provide a desired braking effect. In this case, the hydraulic braking system is operated to cooperate with the regenerative braking system, for providing the braking effect desired by the vehicle operator. The braking force that must be generated by the hydraulic braking system is equal to the desired total braking force minus the regenerative braking force generated by the regenerative braking system. Therefore, the fluid pressure in the wheel brake cylinder of the hydraulic braking system must be controlled to generate the hydraulic braking force which is a difference between the desired braking force and the regenerative braking force. To this end, the fluid pressure in the wheel brake cylinder must be controlled irrespective of the operating state of the brake operating member. This mode of control of the fluid pressure in the wheel brake cylinder will be referred to as "cooperative control mode".
The wheel brake cylinder pressure must be controlled irrespective of the operating state of the brake operating member, in other control modes of the hydraulic braking system, as well as in the cooperative control mode. These other control modes include: anti-lock control mode for preventing an excessive amount of slip of the vehicle wheel during brake application to the vehicle; traction control mode for preventing an excessive amount of slip of the vehicle wheel during acceleration of the vehicle; vehicle stability control mode for improving the running stability of the vehicle; and braking-effect control mode for providing an operator's desired braking effect accurately corresponding to the operating state of the brake operating member.
The hydraulically operated braking system including the wheel brake cylinder, hydraulic pressure source and pressure control valve device described above permits the wheel brake cylinder pressure to be regulated irrespective of the operating state of the brake operating member, and is operable in various control modes such as the cooperative control mode. However, the present inventors have recognized a need for some improvements in the operating reliability of the conventional hydraulic braking system.